Al, the first thing to learn about CSS is that it is only as good as the mark-up it is being applied to. Domino can't easily create HTML mark-up for Rich Text Fields. It does a best guess (in clean mark-up terms, a very bad guess) at displaying what the user has applied for formatting and what the browser can display.
The only way to learn what is going on, and how bad Domino's HTML output can be, is to view the page source from within the browser and see what tags have been generated.
It is important that you learn what good HTML structure is and how HTML should be structured, without this knowledge it is hard t understand how CSS should be applied to the structure.
Remember that CSS is applied to tags and groups of tags so knowing EXACTLY what the mark-up is, is very important to the CSS being applied. If things aren't working you are either targeting the wrong tags or Domino is over-riding your CSS with its own. When dealing with Rich Text Fields, it's usually the latter.
This article and its comment thread might help a bit...
http://www.notestips.com/80256B3A007F2692/1/NAMO5BMVNR
Mike Golding